Carbon fibers (CFs) are materials comprising 92% or more carbon, and having high values in thermal conductivity, specific strength and elasticity, and low values in modulus of elasticity, coefficient of thermal expansion and density. Also, carbon fibers are materials suitable for use in a high temperature structure by virtue of relatively good processability, and in particular, interest in carbon fibers as a reinforcing material for high-tech composites having high processability is increasing. In addition, carbon fibers are highly resistant to thermal shock, and thus have been widely used in various industry fields as materials for an ultra-high temperature structure such as friction materials for aircrafts, nose cones for space shuttles, heat resistant materials for nuclear reactors and nozzles for rockets, which take a great of heat in a very short time.
However, although carbon fibers (CFs) have favorable engineering properties, a surface of carbon fiber are oxidized by reacting with oxygen when exposed to air at a temperature of 500° C. or more, or gases such as CO or CO2 are generated, resulting in an erosion (ablation) of material. Thus, pure carbon fibers are not suitable for use in these applications. That is, as shown in FIG. 1, when carbon fibers are treated by heat (1000° C.) in air, pores are formed on a surface of carbon fiber by CO or CO2 are generated, resulting in an erosion (ablation) of material. Thus, pure carbon fibers are not suitable for use in these applications. That is, as shown in FIG. 1, when carbon fibers are treated by heat (1000° C.) in air, pores are formed on a surface of carbon fiber by CO or CO2 which are oxidized by reacting with oxygen at a high temperature. Further, a decrease in weight and strength of carbon fibers thereby may limit their applications. Then, when a carbon fiber having unstable surface is exposed to an ultra-high temperature of 2000° C. or more, portions attenuated due to oxidation are cut off by thermal conduction, which is called ‘ablation’. Therefore, the surface and structure of carbon fiber as well as intrinsic properties thereof should be protected in an oxygen atmosphere.
As described above, to use carbon fibers at a high temperature, they have to be separated from an oxygen atmosphere to prevent oxidation from being generated. Therefore, to prevent the oxidation of carbon fibers (CFs) at a high temperature and improve heat and ablation resistances thereof, it is very important to ensure a technology for making a composite with carbon fibers.
Generally, there are two approaches to prevent carbon fibers from being oxidized and improve heat and ablation resistances thereof. One approach is a method of adding an oxidation inhibitor to carbon, and the other approach is a method of coating an oxygen-impermeable layer on a surface of carbon fiber. The former can inhibit oxidation at up to about 1000° C. but this method has a disadvantage that an effect of inhibiting oxidation is greatly lowered at a high temperature of 1000° C. or more. Therefore, it may be necessary to form a coating layer to prevent carbon fibers from being oxidized at a high temperature of 1000° C. or more.
Generally, as methods for coating an oxidation protective layer for a carbon fiber, when a temperature in use is about 1000° C. or less, a cheap phosphoric acid-based coating layer is used. When a temperature in use is about 1000° C. or more, boron-based coating layer is used. However, the use of boron is limited due to a high equilibrium vapor pressure of boron at a high temperature of 1500° C. or more.
Therefore, to solve this problem, at a temperature of 1500° C. or more, the oxidation of carbon fiber is inhibited by forming an amorphous SiO2 layer having a low oxygen transmittance rate on a surface of carbon fiber.
U.S. Patent Application Publication No. 2004/0258839 relates to “a method for forming an oxidation protective coating layer to impart oxidation resistance to a carbon/carbon composite”. The above-mentioned patent discloses a method of forming two or more oxidation protective coating layer by impregnating only Si on a carbon/carbon composite using a pack cementation technology, and in particular, a composite coating method capable of controlling the thickness of coating layer by 10 μm to 2,000 μm. However, this method fails to achieve a satisfactory effect in terms of a coating process or fire resistance. Further, the method has a disadvantage that a chemical vapor reaction is used in coating ceramics, and thus a process is complicated and expense.